1. Field of the Invention
The present invention relates to an antenna for RFID (Radio Frequency Identification), which communicates with an external device by means of an electromagnetic field signal, and a portable electronic device having the antenna mounted therein.
2. Description of the Related Art
An electronic money function is realized by installing an RFID function into a portable electronic device such as a cellular phone. In this case, settlement is made through non-contact communication between a portable electronic device with, for example, a reader/writer sited in a shop or the like, or a reader/writer installed in an automatic ticket checking machine or the like for railroads.
In a portable electronic device installed with the RFID function that communicates with an external device by means of an electromagnetic field signal, an antenna coil for RFID is typically provided at a battery retaining section or the like in the device's case, as disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2003-67692, Unexamined Japanese Patent Application KOKAI Publication No. 2004-227046 and Unexamined Japanese Patent Application KOKAI Publication No. 2004-320571.
Unexamined Japanese Patent Application KOKAI Publication No. 2003-67692 discloses the configuration that has a radio communication module integrally mounted in the battery lid of a cellular phone. The radio communication module has an antenna coil, an IC chip connected thereto and an electromagnetic shield member is integrated with the interior of the battery lid.
Unexamined Japanese Patent Application KOKAI Publication No. 2004-227046 discloses an RFID apparatus which communicates with an automatic ticket checking machine using an electromagnetic field. A board having an RFID antenna coil printed thereon is provided at the battery retaining section of a cellular phone, and the antenna coil is arranged on the battery lid side. Specifically, the antenna coil is printed on the board in a rectangular spiral pattern, and an IC and a resonance capacitor which are to be connected to the antenna coil are mounted on the board.
Unexamined Japanese Patent Application KOKAI Publication No. 2004-320571 discloses a configuration where the antenna of an RFID reader/writer or tag is attached to outside a battery in parallel to the surfaces thereof, or a configuration where the antenna is attached to a cover lid. The tag is installed in a prepaid type card and has an antenna coil wound in a spiral form and an IC chip configured to be electrically connected to the antenna coil and to be able to store unique information of a card owner.
The RFID that is realized by such a configuration generally has a characteristic such that the larger the area surrounded by the loop of the antenna coil (coil loop area) or the loop diameter is, the higher the communication distance characteristic becomes.
For example, the RFID system which is used in an automatic ticket checking gate or the like of a public transport such as railroads is demanded of a relatively long communication distance characteristic to permit a user to pass the gate smoothly. In this respect, such the RFID system uses an antenna with a long communication distance or a relatively large loop diameter for the reader/writer, and uses an antenna with a relatively large loop diameter for an electronic train ticket. That is, the RFID system uses an antenna having a strong electromagnetic field characteristic.
On the contrary, an RFID system to be used in an electronic settlement machine or the like which is sited in a shop or the like is demanded of a relatively short communication distance characteristic to prevent erroneous connection. In this respect, such the RFID system uses an antenna with a short communication distance for a reader/writer, or an antenna with a relatively small loop diameter, and an antenna with a relatively small loop diameter for an electronic wallet (prepaid card). That is, the RFID system uses an antenna having a slight electromagnetic field characteristic.
There arises no problem if RFID systems require different antenna characteristics as long as the systems use different RFID cards. When a portable electronic device is equipped with the RFID function, however, plural types of RFID systems can be used in changing an application, thus requiring an antenna compatible with a plurality of RFID systems.
Because the conventional antenna coil used for RFID have a simple structure of having a plurality of turns in a spiral form as shown in FIGS. 14A and 14B, however, it is difficult to make the antenna coil compatible with both of the aforementioned two types of RFID systems.
That is, if the loop diameter of the RFID antenna to be mounted in a portable electronic device is made larger in such a way as to be compatible with an RFID system such as an automatic ticket checking machine, a communication failure may occur when the portable electronic device communicates with a reader/writer which is used for settlement of an electronic money in a shop or the like.
In a loop antenna, while a magnetic field is generated at the conductive portion forming the loop, the magnetic field strength becomes lower at a portion near the center of the loop where the conductive portion is not present. When the loop diameter of the loop antenna of the portable electronic device is larger than that of the loop antenna of the reader/writer, the center portion of the loop antenna of the portable electronic device comes close to the position corresponding to the loop antenna of the reader/writer. In this case, the center portion of the loop antenna has a lower magnetic field, making the amount of the magnetic flux crossing both the loop antenna and the reader/writer smaller.
The use of the load modulation system in RFID makes a load modulation signal weaker when the amount of the magnetic flux is small, generating a non-communicatable area or so-called proximal Null. The generation of such a non-communicatable area disables communications between the portable electronic device and the electronic settlement machine even if the former is placed over the latter.
Accordingly, it is an object of the present invention to provide an antenna and a portable electronic device in an RFID system, which can reliably communicate with a reader/writer for a slight electromagnetic field whose loop antenna has a relatively small loop diameter as well as a reader/writer for a strong electromagnetic field whose loop antenna has a relatively large loop diameter.